Polyesters have become quite an important engineering thermoplastic materials. The two most common polyesters are poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), and, to a lesser extent, poly(cyclohexylenedimethylene terephthalate) (PCT). PET has a higher melting point and higher modulus and is economically more attractive than PBT. However, these types of polyesters suffer from slow crystallization rates making it necessary to use long molding cycles and high mold temperatures when fabricating molded parts. PET along with PCT are particularly deficient in this respect which tends to negate their economic advantages, particularly in regard to PET.
It is known that the crystallization rate and crystallization temperature of PET can be increased by the addition of a nucleating agent to a molding composition. Several nucleating agents have the characteristic of promoting rapid crystallization under conditions of rapid cooling from the melt. An acceptable nucleating agent should be highly effective at low concentration in promoting crystallization, and further should be readily dispersible so that rapid and uniform crystallization of the polymer from the melt is possible.
A number of techniques have been applied to improve the crystallization rate of poly(alkylene terephthalates) by way of nucleating the amorphous polymer. These techniques have involved the seeding or nucleation of the amorphous materials in much the same manner as supersaturated organic solutions are seeded to cause a solute to crystallize out; see “Heterogeneous Nucleation of Polyethylene Terephthalate” by D. Garcia, Polymer Preprints, American Chemical Soc., 25(1), 197 (1984).
Commonly employed nucleating agents for PET are the alkali and alkaline earth metal salts of organic acids. U.S. Pat. No. 3,352,904 discloses the sodium and potassium salts of carboxylic acids, as well as the salts of organic polymers containing pendant acidic groups, including carboxylic acid groups, as nucleating agents for PET. U.S. Pat. Nos. 4,322,335; 4,368,288; 4,305,864; 4,390,493; 4,393,178 and EP 21,648 and 351,732 all disclose the use of ionizable metal salts of aromatic ring containing compounds, wherein the aromatic rings are substituted by acid groups which are carboxylic acid, sulfinic acid, phosphonic acid, phosphinic acid and phenolic groups.
U.S. Pat. No. 3,575,931 shows metal salts of carboxylic acids as nucleating agents for PET. U.S. Pat. No. 4,401,792 discloses alkali metal salts of polyethylene ionomers and benzoic acid as nucleating agents for PET. U.S. Pat. No. 4,425,470 discloses alkali metal salts of ethylene terephthalate oligomers as nucleating agents for PET. U.S. Pat. No. 4,551,507 shows alkali and alkaline earth metal carboxylate salts of tertiary and quaternary amines as PET nucleating agents. Other inorganic nucleating agents are disclosed in U.S. Pat. No. 4,397,979 such as certain lithium, manganese, and zirconium salts.
However, when the need exists for very fast crystallization rates or even lower mold temperatures, merely adding greater quantities of inorganic or organic nucleating agents with low molecular weight is not satisfactory, because the incremental effect of such nucleating agents frequently diminishes with greater quantities of the same, and large quantities of such nucleating agents can have an adverse effect on the physical properties of molded articles made from compositions containing the same. Exemplary drawback of polyester compositions with nucleating agents known to the art is that the polymer during its processing suffers a decrease in weight average molecular weight, mostly because of the degradation phenomenon caused by ionic nucleating agents. Further, small molecular weight acids remaining in the polymer after nucleation which are derived from the acid salts cause acid degradation of the PET.
Therefore, there still remains a need for providing rapidly crystallizing polyester compositions in a facile, economic, and simplified manner, particularly absent or substantially absent inorganic or organic nucleating agents with low molecular weight.